This invention relates to a relatively high temperature furnace for melting thermo-plastic materials. More specifically, the furnace is adapted for melting glass wherein the furnace enclosure is a vessel particularly suited to resist the corrosive effects of the glass at temperatures in excess of 1800.degree. C. Means are described for enhancing thermal efficiency and for protecting the furnace during startup. Other details are also hereinafter described in the specification.
Refractory lined furnaces have been used for many years to melt glass. Many standard refractories, however, have a tendency to become slowly dissolved or corroded by the glass until the furnace begins to leak. The rate of this corrosion increases rapidly with increasing temperature and glass fluidity. While repairs may be made, they are usually difficult to effect, expensive, and usually short lived. The refractory may be cooled on the outside surface to slow down this corrosion, but at a cost of higher energy losses.
The most severe corrosion usually occurs at the sidewalls near the top of the glass bath. In conventional furnaces the glass is hottest near the top, and melting and refining temperatures are limited by the refractory capabilities to less than 1600.degree. C. As the refractory dissolves into the glass many of the corrosion products are swept into the molten bath. The dissolved refractory materials become part of the glass composition and in some cases may have a deleterious effect on glass quality. The heavier corrosion products tend to sink to the bottom of the furnace and form a somewhat loosely arranged protective layer for the bottom wall.
Another area where the refractory corrosion tends to be high is at the throat or exit portion of the furnace. Many times such exit areas are clad with metals for protection. In the disclosure of Spremulli, U.S. Pat. No. 4,029,887 a molybdenum (moly) pipe was used to provide a highly corrosion resistant conduit from a furnace to a forehearth channel. Platinum too, has been used for exit liners. In fact entire furnaces may be platinum lined, but at extremely high cost.
In vertical electrical melting units, an example of which is disclosed in U.S. Pat. No. 3,524,206, the top of the molten bath is covered with a cold batch blanket. Corrosion in this type of furnace is typically most severe in the upstanding sidewall near the so-called fusion line and around the electrodes entering through the sidewall. The present invention provides means for substantially reducing such corrosion and/or minimizing its effect.
In conventional vertical electric melters, having a cold batch blanket, there is a tendency to retain seeds in the glass since there is no free surface to allow for the rapid escape of bubbles trapped in the glass. Therefore residence time of glass in the furnace must be regulated to assure sufficient fining. Since freshly melted glass tends to move quickly toward the exit before it can be refined, the fast moving glass sets up unwanted convection currents which contribute to furnace deterioration. Thus, steps must be taken to control convection currents and increase the residence time of the glass in the furnace. One such method is described in U.S. Pat. No. 4,143,232, wherein controlled convection currents are produced by deeply immersed electrodes activated in a selected firing arrangement. Another advantage of the latter arrangement is that the heat produced is concentrated away from the walls, thus reducing corrosion around the electrode openings therein. In the present invention an improved arrangement of electrodes is adapted to provide concentrated central heating of the glass and hot spot fining.
Molybdenum, platinum, platinum alloys, and to some extent steel alloys and iron have long been recognized as materials having a higher resistance to wear than conventional refractory and are considered useful in the construction of glass melting furnaces. Molybdenum, for example, has been used as an electrode material and as a lining for stirrer wells where high glass velocities produce rather severe corrosion. As mentioned above, furnace outlets are often lined with platinum and sometimes molybdenum.
Platinum is extremely expensive and its use is often limited to the melting of special glasses such as ophthalmic or optical glasses. Iron may be used, as disclosed in British Pat. No. 601,851, but it has a relatively low melting point and it can contaminate most glasses with colorants. For certain purposes, however, it may be an acceptable furnace liner material.
Moly is recognized as a metal that has high temperature strength, is relatively inexpensive, and is chemically compatible with many glasses. A distinct disadvantage of this material is that it will oxidize above 550.degree. C. In the past it has been difficult to fabricate. Now that moly can be formed into flat or curved plate and pipe and welded into structures, it is a more attractive material. One of the most extraordinary advantages of moly, which melts at 2600.degree. C., is its high temperature strength which allows it to be used up to about 2200.degree. C. Note for example that platinum, which has heretofore been used almost exclusively in high temperature work, melts at 1730.degree. C. and can be used up to only about 1600.degree. C. Thus, moly is an extremely useful material since it is substantially less expensive than platinum and has a much higher melting point.
The U.S. Patent to Silverman, No. 3,109,045, suggests the use of molybdenum as a vessel material in a glass melting furnace. A molybdenum crucible portion is submerged in an external bath of thermoplastic material to protect the exterior portion thereof from oxidation. The interior portion of the crucible is filled with molten thermoplastic material, thus the moly is protected from the ambient atmosphere and will not oxidize. Further, although the exterior of the moly crucible is protected by glass, a refractory tank or containment vessel for the exterior bath into which the moly crucible is located is large in comparison to the latter. Thus, the molten glass surrounding the vessel will have freedom to convect and ultimately destroy the refractory containment vessel.
The Silverman unit is of a size and configuration adapted for specialty melts and would be impractical to scale up. In addition it requires a purge gas arrangement to remove air from the batch materials during operation for the purpose of protecting the upper portion of the moly vessel from oxidation. Also, since the batch materials for most glasses will contain oxidizing agents such as CO.sub.2, SO.sub.2 and H.sub.2 O, the batch cannot be allowed to contact the moly. On the other hand, if the glass level is maintained above the moly, it will contact the refractory ring which sits on top of the moly, thus causing the refractory to quickly corrode.
Gas firing of batch materials would be difficult to implement in a moly furnace without deleterious effects because the heat and oxygen in the flame is highest at the glass surface, precisely where protection against corrosion and oxidation is needed. Thus, without the precautions hereinafter suggested by the present invention, a moly liner would oxidize since it would be exposed to the combustion gases.
Joule heating is a preferred method of melting glass in a furnace of the type described herein, especially a moly lined furnace. However, since molybdenum is a conductive metal, one must place the electrodes in selected locations and provide appropriate circuitry in order to optimize current flow in the glass. While it is normally desirable to avoid a short circuit to the liner, it may be desirable to place the electrodes and provide circuitry so that some current flows to the liner for providing uniform power dissipation. Moreover, it is possible to fire directly to the liner if desired. Batch electrodes may be suitable for this purpose and various arrangements are illustrated in U.S. Pat. Nos. 2,215,982, 2,978,526 and 4,159,392. In a preferred arrangement of the present invention it is contemplated to use movable batch electrodes. While the '526 patent discloses such a concept, the arrangement is limited in flexibility and would drastically interfere with the proper filling of the furnace.
In a series of related U.S. Pat. Nos. 4,351,664; 4,352,687 and 4,365,986 assigned to the assignee herein, various arrangements of glass transport and conditioning systems useful with the present invention are disclosed in detail. It should be understood that, to the extent necessary, the teachings of said applications should be considered incorporated herein by reference.